De#initions I. Energy ( ): II. Heat ( ): A. Heat is not a substance. Objects do not contain heat, they B. Molecules with each other. III. Reaction perspectives: A. System: B. Surroundings: IV: Heat changes: A. Exothermic: Energy is The of the while the of the B. Endothermic: Energy is The of the while the of the V. Work: an object a certain. A. Can be converted to heat energy. B. Includes all forms of energy transfer other than heat transfer. VI. Units: A. VII. State function: A. The property of a system determined by B. Independent of how the system reached that state. C. Examples: D. These are measured using the change from a system's initial and Pinal states. Measuring Heat Flow Property Exothermic Endothermic Energy Released (-) Absorbed (+) System temp. Decreases (-) Increases (+) Surr. temp. Increases (+) Decreases (-) I. Heat capacity (C): A. Units: or B. SpeciPic heat ( ): C. Molar heat capacity ( ): II. Calorimetry:. A. Equation: or 1. 2. 3. 4. B. First law of thermodynamics ( ):
C. cwater = or III. Measured using a A. Coffee cup calorimeter ( ) B. Bomb calorimeter ( ) C. In calorimetry calculations, always take the (unless noted). Example (a) How much heat is needed to warm 250 g of water (about 1 cup) from 22 C (about room temperature) to near its boiling point, 98 C? The specipic heat of water is 4.18 J/g-K. (b) What is the molar heat capacity of water? Practice (a) Large beds of rocks are used in some solar-heated homes to store heat. Assume that the specipic heat of the rocks is 0.082 J/g-K. Calculate the quantity of heat absorbed by 50.0 kg of rocks if their temperature increases by 12.0 C. (b) What temperature change would these rocks undergo if they emitted 450 kj of heat? Answers: (a) 4.9 10 4 J, (b) 110 K = 110ºC decrease Enthalpy I. Enthalpy (H) : A. State function: we care about change in H (ΔH), not H because B. At constant pressure,
1. Sign of ΔH ( ) a. b. 2. (2nd law). 3. It is ( ) (1st law). C. A change of state involves a. H2O(s) --> H2O(l) ΔH= D. Enthalpy of reaction: measures the ΔH of a reaction. 1. Example: How much heat is released when 4.50 g of methane gas is burned in a constant-pressure system? Hydrogen peroxide can decompose to water and oxygen by the following reaction: Calculate the value of q when 5.00 g of H2O2(l) decomposes at constant pressure. Answer: 14.4 kj II. Hess's Law (3rd Law) A. States that the B. Thus, C. When calculating... 1. 2.
1. The enthalpy of reaction for the combustion of C to CO2 is 393.5 kj/mol C, and the enthalpy for the combustion of CO to CO2 is 283.0 kj/mol CO: Using these data, calculate the enthalpy for the combustion of C to CO: Carbon occurs in two forms, graphite and diamond. The enthalpy of the combustion of graphite is 393.5 kj/mol and that of diamond is 395.4 kj/mol: Calculate ΔH for the conversion of graphite to diamond: Answer: ΔH3 = +1.9 kj 2. Calculate ΔH for the reaction given the following chemical equations and their respective enthalpy changes:
Calculate ΔH for the reaction given the following information: Answer: 304.1kJ III. Enthalpy of formation ( ) A. MEMORIZE: B. Dependent on C. New thermo standard!!! 1. Standard state ( ): 2. If standard state is mentioned, 3. Standard enthalpy of formation ( ): D. Many ΔH f values have been measured so you can Pind ΔH using ΔH f values and Hess's Law. E. REMEMBER: F. NOTE: the ΔH f for (ex. ) G. Mind your H. Note: an equation of formation needs to have I. Refer to p. A26-A33 Calculate the standard enthalpy change for the combustion of 1 mol of benzene, C6H6(l), to CO2(g) and H2O(l).
The standard enthalpy change for the reaction is 78.1 kj. From the values for the standard enthalpies of formation of CaO(s) and CO2(g) given calculate the standard enthalpy of formation of CaCO3(s). IV. Enthalpy of combustion ( ) A. MEMORIZE: B. Also dependent on C. Standard enthalpy of combustion ( ): D. ΔHc involves E. You will need to remember how to write combustion reactions: Ex... V. Bond enthalpies (units: ) A. DePinition: B. Refer to Table 8.8 on p. 329. C. Same concept: D. E. Example: Estimate ΔH for the following reaction (where we explicitly show the bonds involved in the reactants and products): Estimate ΔH for the reaction Answer: 86 kj F. Bond length revisited 1. You can Pind the bond length and bond energy using the graph on the right.
VI. Potential Energy Diagrams 1. Remember that potential energy and kinetic energy are related. ( ) 2. Let s analyze the diagram below. VII. Born-Haber Cycle (a neat summary...) A. Thermochemical cycle used to analyze stability of ionic compounds. B. Consider: Na(s) + ½Cl2 (g)-->nacl(s) ΔH=-411kJ C. FYI: Na(s) --> Na(g) ΔH=108kJ ½Cl2 (g) --> Cl(g) ΔH=122kJ Na(g) --> Na + (g) + e - ΔH=496kJ (1st IE) Cl(g) + e - --> Cl - (g) ΔH=-349kJ (EA) D. What is ΔHlattice? Na+(g) + Cl-(g)-->NaCl(s)